1. Field of the Invention
The present invention relates in general to phonograph record turntables, and more particularly to a detector for distinguishing between recorded and unrecorded portions of a record carried on the turntable. Such a detector has application in an automatic tune selection apparatus that detects and initiates reproduction of a desired recorded band from among a plurality of recorded bands on the phonograph record surface.
2. Description of the Prior Art
U.S. Pat. No. 2,952,464, issued to Stimler, describes a detector located on the tone arm of a phonograph for sensing the difference in amplitude of light reflected from areas of the record surface as the tone arm scans above the record. Such detector utilizes a light projector and a first lens to form a converging beam of light that illuminates a portion of the record surface, and then utilizes a second lens to focus light reflected from the record surface onto a photoelectric cell. While such a detector focuses light from the record surface onto a photocell, as a practical matter light reflected from records having differing thickness, material composition, manufacturing tolerances and warping cannot be detected with the accuracy desired to provide a reliable detector.
Nakagiri, U.S. Pat. No. 3,368,080, recognizes the shortcomings of Stimler and attempts to solve the problem with a detector utilizing parallel light beam illumination of the record surface and having no lens in the reflected light path. However, a good parallel light beam source at reasonable cost is impractical. Therefore, Nakagiri added to the reflected light path a thick dual parallel slit mask in conjunction with dual differential photocells and a Schmitt trigger circuit.